Industrial methods for preparing polyolefin from olefin include a solution polymerization process, a slurry polymerization process and a gas-phase polymerization process. In the solution polymerization process, polymers are produced in a dissolved state in a solvent. In the slurry polymerization process, polymers of solid state are produced and dispersed in a liquid medium. In the gas-phase polymerization process, produced polymers are dispersed in a fluidized state in a gas medium. Generally, in the gas-phase polymerization process, polymerization is carried out by using a bubbling fluidized bed polymerization reactor.
Due to the development of metallocene catalysts having superior catalytic activity and selectivity, industrial-scale processes for preparing polyolefin from alpha (α)-olefin under the solid catalyst in a gas phase medium are widely used. In the gas-phase polymerization method, in order to maintain a polymer bed through which reaction gas flows, the reaction bed is mechanically stirred (stirred bed reactor) or a reaction gas is continuously circulated to fluidize the reaction bed in a suspension state (fluidized bed reactor). In the stirred bed reactor and the fluidized bed reactor, the composition of monomers around produced polymer particles remains constant by an induced stirring and is maintained like an ideal state of a Continuous Stirred Tank Reactor (CSTR). Thus, the reaction condition can be easily controlled, and products of uniform quality can be obtained under a steady-state condition.
Nowadays, the most common industrial gas-phase polymerization method utilizes a fluidized bed reactor operated under a “bubbling” condition. FIG. 1 shows an internal circulating (circulation type) fluidized bed polymerization reactor (bubbling fluidized bed reactor 10) where the conventional polymerization method is carried out. As shown in FIG. 1, in the conventional bubbling fluidized bed reactor 10, circulation gas flows into the reactor through circulation gas inlet 11 and a gas distributor (distribution plate) 12, and polymerized by contacting with polyolefin particles in the reactor 10, and the unreacted circulation gas is discharged through an outlet 13. Catalysts and/or pre-polymers can be introduced into the reactor 10 through a catalyst or pre-polymer inlet 14. The bubbling fluidized bed polymerization reactor has superior heat removability and homogeneous temperature distribution compared with other polymerization reactors. Thus, the composition in the reactor remains constant, and reactants of solid state (i.e., polymer) can be stayed in the reactor for a relatively long time, and the reactants of solid state flows like fluid in the reactor, which allows easy handling of the polymer. Beside these various advantages, there is another advantage in that operation and design of the reactor are easy because of its simple structure.
However, it is difficult to increase the contact frequency and the contact time between the catalysts and the reactants in the bubbling fluidized bed. This limits the productivity per unit volume of the reactor. To solve these problems, U.S. Pat. No. 5,834,571 and U.S. Pat. No. 5,436,304 disclose methods for increasing productivity by injecting condensation-inducing materials into the circulation gas, and by directly injecting liquid into the bubbling fluidized bed. However, the method of changing the composition of the circulation gas causes a drastic change of the reaction condition, and additional costs are necessary for mounting a pump to inject the condensation-inducing materials and a reservoir for condensate.
As shown in FIG. 1, in the conventional method, polymers exist within a vertical cylindrical area of reactor 10, and reaction gas discharged through the outlet 13 of reactor 10 is compressed and cooled by a compressor, and recycled to a lower part of the polymer bed through the inlet 11 of the reactor 10, preferably, with makeup monomers and a suitable amount of hydrogen gas. In the gas medium, entrainment of the solid polymer is prevented by suitably designing the upper part of the reactor 10 to provide a freeboard, namely, a space between the surface of the polymer bed and the gas outlet. That is, the entrainment of the solid polymer is controlled as the velocity of the gas in upper part of the reactor 10 is reduced. In some methods, the velocity of the gas is reduced by installing a cyclone in the gas outlet line. The flow velocity of the circulation gas is controlled within the range between “a minimum fluidizing velocity” and “a transport velocity”. The heat of reaction is removed by cooling the circulation gas, or can be controlled by adding inert gas. Reactor is usually controlled at constant pressure of 1 to 3 MPa, and catalysts are continuously supplied, and composition of polymer is controlled according to the composition of the gas phase. Hereinafter, the fluidized bed for the gas-phase polymerization is explained in more detail.
(A) Removal of Heat of Reaction
In the gas-phase polymerization reaction, the maximum fluidizing velocity of the circulation gas is limited very narrowly, and the freeboard volume larger than the volume of the fluidized bed is necessary, and the temperature of the gas inlet is maintained preferably higher than the dew point of the gas mixture. The productivity of the reactor (production amount per hour for a unit cross section of the reactor) is limited according to a heat of reaction, polymer dimension, and a gas density. Especially, in case of preparing a copolymer of ethylene and higher α-olefin (for example, hexene, octene) by using a conventional Ziegler-Natta catalyst, productivity may be decreased. Methods to control the inner temperature of the reactor and remove the heat of reaction by using a partial condensation of the circulation gas and latent heat of evaporation of condensate were provided (European patent No. 89691, U.S. Pat. No. 5,352,749, and international patent publication No. WO 94/28032), but the operation of the fluidized reactor is very important in these methods. In European patent No. 89691 and U.S. Pat. No. 5,352,749, a turbulence, which is produced by a grid which distributes liquid onto polymers, is used. However, if polymers are sticky, uncontrollable phenomena may occur such as a formation of agglomerates or an inequality of liquid distribution because of a cohesion in plenum. Moreover, in the methods, problems relates to a distribution of wettable solids may arise in the plenum. And, the identification standard mentioned in U.S. Pat. No. 5,352,749 is appropriate in a steady state, but cannot offer desirable solutions to temporary “abnormal reaction” situation which can cause irreversible loss of fluidization bed and the accompanying shutdown of a reactor. In the mentioned method of international patent publication No. WO 94/28032, the heat of reaction is controlled by separating agglomerates and by using dimension of a nozzle and a grid. Actually, in reaction condition, agglomerates contain solids so that the less agglomerates, the higher concentration of the solid, and efficiency depend on violent circulation of solids in the reactor. However, it is difficult to disperse a suspension uniformly into a number of nozzles, and if one of the nozzles is clogged, liquid evaporated from the relevant part is unevenly distributed and an imbalance in gas flow velocity occurs by plenty of agglomerates so that efficiency may be deteriorated. Moreover, the reactor must be completely stopped when the nozzles are repaired.
(B) Discharge of Products
The most simple method to discharge polymers from a reactor is to directly discharge polymers from the fluidized bed by a control valve. This method has advantages of no generation of stagnant zone and simplicity. If the pressure at the lower stream of the discharging valve is maintained low (0.5 to 3 bar gage), monomers dissolved in polymers are evaporated, or partial pressures of the monomers in gas become low, and the temperature falls, and thus the reaction is stopped actually. However, the amount of gas discharged with polymers through an orifice from the fluidized bed depends on the pressure of reactor, a fluidizing velocity, a density of solid in the bed, and so on, and is generally large. Thus, it is necessary for the gas to be recompressed and returned to the reactor from the collector because the large amount of gas discharged with polymers increases the production costs and operation costs. Therefore, discontinuous discharging systems with two or more hoppers operated alternately are used in many industrial reactors. For example, U.S. Pat. No. 4,621,952 discloses a discharging system in which polymers are intermittently transferred from a reactor to a temporary (stationary) tank by high differential pressure. In the filling stage, momentum of polymers effects on the wall of a temporary tank and then on the polymer bed, and densifies polymer particles and takes fluidity from the polymers. In filling stage, inner pressure of the temporary tank quickly increases to the pressure of the reactor, and temperature is maintained. However, polymerization reaction is adiabatically progressed at high speed so that soft and adhesive products become agglomerates which cannot be granulated, and it becomes difficult to discharge them to a collecting tank. Similar problems may happen in the method of U.S. Pat. No. 4,703,094. Complex continuous systems are being developed because of the shortcomings of the intermittent systems. For example, a screw is installed inside a reactor to densify and discharge polymers in Japanese patent Laid-open No. 58-032634, and an extruder is installed inside a fluidized bed reactor in U.S. Pat. No. 4,958,006. However, these methods are complex and not suitable for an industrial application, and it is difficult to supply polymers to a next reaction stage.